A sign made of a vertical line of LEDs?

Several times in the past I've seen home-brewed and perhaps CPU-driven signs that consist of a single vertical line of perhaps a dozen or two LEDs. The "sign" doesn't appear to be anything but a line of glowing LEDs. Only when you view it at a glance, as your vision moves quickly from side-to-side, do you see that it is rapidly flickering the vertical scanlines of a simple dot-matrix image: maybe the digits of the time, perhaps a smiley face, or some other simple image. Anyone here know the name of this sort of device, so I can web-search for others who've made one? It wouldn't be hard to craft. To be obliquely on-topic, I've thought about making a simple circuit that would be driven by the parallel port of an old PC. - John